ccache: change default cache directory path to match config setting

Commit 43329076 changed the hard-coded
ccache directory location to use BR_CACHE_DIR (then BUILDROOT_CACHE_DIR),
which is exported by Makefile based on the BR2_CCACHE_DIR config option.
This allowed the cache location to be changed on-the-fly by setting a
"make" command line variable, but left the default location of ccache's
normal default at "$HOME/.ccache".  Since this location does not match the
default for BR2_CCACHE_DIR, it is basically almost never correct, so
direct invocation of ccache outside of the buildroot Makefile, such as for
increasing the cache size, becomes cumbersome.

This patch changes the last-ditch cache location from "$HOME/.ccache" to
the BR_CCACHE_DIR value defined when host-ccache is configured.  Note that
the ability to later override the cache location by using a BR_CACHE_DIR
command line variable is left intact.

Signed-off-by: Danomi Manchego <danomimanchego123@gmail.com>
Reviewed-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Tested-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>